Polyketides represent a large family of diverse compounds synthesized from 2-carbon units through a series of condensations and subsequent modifications. Polyketides occur in many types of organisms, including fungi and mycelial bacteria, in particular, the actinomycetes. There is a wide variety of polyketide structures, and the class of polyketides encompasses numerous compounds with diverse activities. Tetracycline, erythromycin, FK506, FK520, narbomycin, picromycin, rapamycin, spinocyn, and tylosin, are examples of such compounds. Given the difficulty in producing polyketide compounds by traditional chemical methodology, and the typically low production of polyketides in wild-type cells, there has been considerable interest in finding improved or alternate means to produce polyketide compounds. See PCT publication Nos. WO 93/13663; WO 95/08548; WO 96/40968; WO 97/02358; and WO 98/27203; U.S. Pat. Nos. 4,874,748; 5,063,155; 5,098,837; 5,149,639; 5,672,491; and 5,712,146; Fu et al., 1994, Biochemistry 33: 9321-9326; McDaniel et al., 1993, Science 262: 1546-1550; and Rohr, 1995, Angew. Chem. Int. Ed. Engl. 34(8): 881-888, each of which is incorporated herein by reference.
Polyketides are synthesized in nature by polyketide synthase (PKS) enzymes. These enzymes, which are complexes of multiple large proteins, are similar to the synthases that catalyze condensation of 2-carbon units in the biosynthesis of fatty acids. PKS enzymes are encoded by PKS genes that usually consist of three or more open reading frames (ORFs). Two major types of PKS enzymes are known; these differ in their composition and mode of synthesis. These two major types of PKS enzymes are commonly referred to as Type I or "modular" and Type II "iterative" PKS enzymes.
Modular PKSs are responsible for producing a large number of 12, 14, and 16-membered macrolide antibiotics including methymycin, erythromycin, narbomycin, picromycin, and tylosin. These large multifunctional enzymes (&gt;300,000 kDa) catalyze the biosynthesis of polyketide macrolactones through multistep pathways involving decarboxylative condensations between acyl thioesters followed by cycles of varying .beta.-carbon processing activities (see O'Hagan, D. The polyketide metabolites; E. Horwood: New York, 1991, incorporated herein by reference). The modular PKS are generally encoded in multiple ORFs. Each ORF typically comprises two or more "modules" of ketosynthase activity, each module of which consists of at least two (if a loading module) and more typically three or more enzymatic activities or "domains."
During the past half decade, the study of modular PKS function and specificity has been greatly facilitated by the plasmid-based Streptomyces coelicolor expression system developed with the 6-deoxyerythronolide B (6-dEB) synthase (DEBS) genes (see Kao et al., 1994, Science, 265: 509-512, McDaniel et al., 1993, Science 262: 1546-1557, and U.S. Pat. Nos. 5,672,491 and 5,712,146, each of which is incorporated herein by reference). The advantages to this plasmid-based genetic system for DEBS were that it overcame the tedious and limited techniques for manipulating the natural DEBS host organism, Saccharopolyspora erythraea, allowed more facile construction of recombinant PKSs, and reduced the complexity of PKS analysis by providing a "clean" host background. This system also expedited construction of the first combinatorial modular polyketide library in Streptomyces (see PCT publication No. WO 98/49315, incorporated herein by reference).
The ability to control aspects of polyketide biosynthesis, such as monomer selection and degree of .beta.-carbon processing, by genetic manipulation of PKSs has stimulated great interest in the combinatorial engineering of novel antibiotics (see Hutchinson, 1998, Curr. Opin. Microbiol. 1: 319-329; Carreras and Santi, 1998, Curr. Opin. Biotech. 9: 403-411; and U.S. Pat. Nos. 5,712,146 and 5,672,491, each of which is incorporated herein by reference). This interest has resulted in the cloning, analysis, and manipulation by recombinant DNA technology of genes that encode PKS enzymes. The resulting technology allows one to manipulate a known PKS gene cluster either to produce the polyketide synthesized by that PKS at higher levels than occur in nature or in hosts that otherwise do not produce the polyketide. The technology also allows one to produce molecules that are structurally related to, but distinct from, the polyketides produced from known PKS gene clusters. It has been possible to manipulate modular PKS genes other than the narbonolide PKS using generally known recombinant techniques to obtain altered and hybrid forms. See, e.g., U.S. Pat. Nos. 5,672,491 and 5,712,146 and PCT publication No. WO 98/49315. See Lau et al., 1999, "Dissecting the role of acyltransferase domains of modular polyketide synthases in the choice and stereochemical fate of extender units" Biochemistry 38(5):1643-1651, and Gokhale et al., Apr. 16, 1999, Dissecting and Exploiting Intermodular Communication in Polyketide Synthases", Science 284: 482-485.
The present invention provides methods and reagents relating to the modular PKS gene cluster for the polyketide antibiotics known as narbomycin and picromycin. Narbomycin is produced in Streptomyces narbonensis, and both narbomycin and picromycin are produced in S. venezuelae. These species are unique among macrolide producing organisms in that they produce, in addition to the 14-membered macrolides narbomycin and picromycin (picromycin is shown in FIG. 1, compound 1), the 12-membered macrolides neomethymycin and methymycin (methymycin is shown in FIG. 1, compound 2). Narbomycin differs from picromycin only by lacking the hydroxyl at position 12. Based on the structural similarities between picromycin and methymycin, it was speculated that methymycin would result from premature cyclization of a hexaketide intermediate in the picromycin pathway.
Glycosylation of the C5 hydroxyl group of the polyketide precursor, narbonolide, is achieved through an endogenous desosaminyl transferase to produce narbomycin. In Streptomyces venezuelae, narbomycin is then converted to picromycin by the endogenously produced narbomycin hydroxylase. (See FIG. 1) Thus, as in the case of other macrolide antibiotics, the macrolide product of the narbonolide PKS is further modified by hydroxylation and glycosylation. FIG. 1 also shows the metabolic relationships of the compounds discussed above.
Picromycin (FIG. 1, compound 1) is of particular interest because of its close structural relationship to ketolide compounds (e.g. HMR 3004, FIG. 1, compound 3). The ketolides are a new class of semi-synthetic macrolides with activity against pathogens resistant to erythromycin (see Agouridas et al., 1998, J Med. Chem. 41: 4080-4100, incorporated herein by reference). Thus, genetic systems that allow rapid engineering of the narbonolide PKS would be valuable for creating novel ketolide analogs for pharmaceutical applications. Furthermore, the production of picromycin as well as novel compounds with useful activity could be accomplished if the heterologous expression of the narbonolide PKS in Streptomyces lividans and other host cells were possible. The present invention meets these and other needs.